1. Field of the Invention
The present invention relates to an optometric apparatus for examining (measuring) refractive power of an eye of an examinee subjectively.
2. Description of Related Art
There is known an optometric apparatus in which optical elements such as a spherical lens and a cylindrical lens are arranged in a test window provided in front of an eye of an examinee, and a forward-presented optotype is shown via the arranged optical elements for subjectively examining (measuring) the eye. And such apparatus is proposed that a plurality of spherical lenses with different power are arranged in three disks for spherical lenses, and the disks are rotated respectively to change a combination of the spherical lenses which are to be arranged in the test window so that spherical power (spherical refractive power) is applied to the eye. Also, proposed is an apparatus such that a plurality of cylindrical lenses with different power are arranged in two disks for cylindrical lenses (arranged in further positions than (posterior to) the disks for spherical lenses from the eye), and the disks are rotated respectively to change a combination of the cylindrical lenses which are to be arranged in the test window so that cylindrical power (cylindrical refractive power) is applied to the eye. And, a lens diameter of the optical elements used for such apparatus is about Φ20 mm (i.e. an effective diameter is about Φ19 mm)
Incidentally, when the spherical power and the cylindrical power, which are generated by combining a plurality of the spherical lenses and the cylindrical lenses, are applied to the eye, a difference between examination power, which is the simple sum of the power of the respective combined lenses, and resultant power through conversion for a spectacles wearing reference position needs to fall within a predetermined permissible difference. However, even though a difference between lens power and power through conversion for the spectacles wearing reference position is arranged to fall within the respective permissible difference in respect to each of the plurality of the lenses (single lens), when they are combined, the difference between the examination power and the resultant power through conversion for the spectacles wearing reference position becomes large. For example, in the case of a configuration in which a plurality of the spherical lenses with different power are arranged in the three (or more) disks for spherical lenses, the spherical lens with middle power (for example, power with an absolute value of 3D or more) is arranged also in the disk which is secondly closer to the eye. Therefore, when the disks for cylindrical lenses are arranged posterior to the three disks for spherical lenses (with respect to the eye), a difference between examination cylindrical power and resultant cylindrical power through conversion for a spectacles wearing reference position becomes large and does not fall within the permissible difference under the influence of refractive power of the spherical lenses arranged secondly and thirdly closer to the eye. Under such circumstances, an examination of high precision may not be performed.